Tests were ordered on all Japanese nuclear plants after the Fukushima disaster. Photograph: Kyodo/Reuters

The plant’s operator, Tokyo Electric Power [Tepco] was forced to increase the amount of cooling water being injected into the No 2 reactor after its temperature soared to 73.3C earlier this week.

By Tuesday night, the temperature had dropped to 68.5C at the bottom of the reactor’s containment vessel, where molten fuel is believed to have accumulated after three of Fukushima Daiichi’s six reactors suffered meltdown after last year’s tsunami disaster.

The temperature at the bottom of the No 2 reactor vessel had risen by more than 20C in the space of several days, although it remained below the 93C limit the US nuclear regulatory commission sets for a safe state known as cold shutdown. Tepco said it had also injected water containing boric acid to prevent a nuclear chain reaction known as re-criticality.

The operator said the sudden rise in temperature did not call in to question the government’s declaration in December that all three damaged reactors had achieved cold shutdown.

“The temperature of the reactor pressure vessel seems to be close to peaking out,” Junichi Matsumoto, a Tepco spokesman, told reporters.

Late last year, however, the minister in charge of the response to the Fukushima disaster, Goshi Hosono, conceded that officials had no idea about the exact location of molten uranium fuel but assumed that it had come to rest at the bottom of its containment vessels.